Dissociation of a complex of F-actin and H-meromyosin by addition of adenosine triphosphate

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Interaction of myosin filaments and minifilaments with actin: a comparative study

Various aspects of actin-myosin interaction were investigated using myosin in the form of filaments and minifilaments obtained by dialysis against citrate-Tris buffer or by adding this buffer to performed myosin filaments. Considerable similarities in the behaviour of the two systems were found. (1) Although the minifilaments are soluble structures, they form insoluble complexes with actin, which superprecipitate upon addition of MgATP. Observations in the electron microscope and from centrifugation experiments have shown that the two actomyosin systems undergo essentially similar structural changes during superprecipitation. (2) At low substrate concentrations the rate of ATP hydrolysis in both systems declines with time, which is typical of insoluble superprecipitating actomyosin. (3) In contrast to soluble myosin subfragments, both filamentous and minifilamentous myosin give biphasic actin-activation curves. (4) The Mg2+-ATPase activities of myosin minifilaments and standard myosin preparations at low KCl extrapolate to similar Vmax at infinite actin concentration. Since our values of Vmax for myosin filaments and minifilaments are in the range of those reported for myosin subfragments, the results of this investigation confirm the view that the catalytic properties of myosin subfragments and intact myosin are equivalent. Moreover, the data show that the extent of myosin aggregation in the initial preparations has no appreciable effect on the characteristic features of the interaction between intact myosin and actin at pH 8.

Actin-myosin interaction: inhibition of the myosin adenosine triphosphatase by actin

In the absence of magnesium ion, the addition of actin to myosin in a 1 :4 ratio has a strong inhibitory effect on the adenosine triphosphatase activity, in contrast to the well-known activating effect of actin in the presence of magnesium ion. This finding suggests that both effects result from a conformational change in the active site of the myosin adenosine triphosphatase.